Re: [PATCH v2] drm/scheduler: Fix hang when sched_entity released

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 25.02.21 um 17:03 schrieb Andrey Grodzovsky:
>
> On 2021-02-25 2:53 a.m., Christian König wrote:
>> Am 24.02.21 um 16:13 schrieb Andrey Grodzovsky:
>>> Ping
>>
>> Sorry, I've been on vacation this week.
>>
>>>
>>> Andrey
>>>
>>> On 2021-02-20 7:12 a.m., Andrey Grodzovsky wrote:
>>>>
>>>> On 2/20/21 3:38 AM, Christian König wrote:
>>>>>
>>>>>
>>>>> Am 18.02.21 um 17:41 schrieb Andrey Grodzovsky:
>>>>>>
>>>>>> On 2/18/21 10:15 AM, Christian König wrote:
>>>>>>> Am 18.02.21 um 16:05 schrieb Andrey Grodzovsky:
>>>>>>>>
>>>>>>>> On 2/18/21 3:07 AM, Christian König wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Am 17.02.21 um 22:59 schrieb Andrey Grodzovsky:
>>>>>>>>>> Problem: If scheduler is already stopped by the time 
>>>>>>>>>> sched_entity
>>>>>>>>>> is released and entity's job_queue not empty I encountred
>>>>>>>>>> a hang in drm_sched_entity_flush. This is because 
>>>>>>>>>> drm_sched_entity_is_idle
>>>>>>>>>> never becomes false.
>>>>>>>>>>
>>>>>>>>>> Fix: In drm_sched_fini detach all sched_entities from the
>>>>>>>>>> scheduler's run queues. This will satisfy 
>>>>>>>>>> drm_sched_entity_is_idle.
>>>>>>>>>> Also wakeup all those processes stuck in sched_entity flushing
>>>>>>>>>> as the scheduler main thread which wakes them up is stopped 
>>>>>>>>>> by now.
>>>>>>>>>>
>>>>>>>>>> v2:
>>>>>>>>>> Reverse order of drm_sched_rq_remove_entity and marking
>>>>>>>>>> s_entity as stopped to prevent reinserion back to rq due
>>>>>>>>>> to race.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>>>>>>>>> ---
>>>>>>>>>>   drivers/gpu/drm/scheduler/sched_main.c | 31 
>>>>>>>>>> +++++++++++++++++++++++++++++++
>>>>>>>>>>   1 file changed, 31 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
>>>>>>>>>> b/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>>>> index 908b0b5..c6b7947 100644
>>>>>>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>>>> @@ -897,9 +897,40 @@ EXPORT_SYMBOL(drm_sched_init);
>>>>>>>>>>    */
>>>>>>>>>>   void drm_sched_fini(struct drm_gpu_scheduler *sched)
>>>>>>>>>>   {
>>>>>>>>>> +    int i;
>>>>>>>>>> +    struct drm_sched_entity *s_entity;
>>>>>>>>>
>>>>>>>>> BTW: Please order that so that i is declared last.
>>>>>>>>>
>>>>>>>>>>       if (sched->thread)
>>>>>>>>>>           kthread_stop(sched->thread);
>>>>>>>>>>   +    /* Detach all sched_entites from this scheduler once 
>>>>>>>>>> it's stopped */
>>>>>>>>>> +    for (i = DRM_SCHED_PRIORITY_COUNT - 1; i >= 
>>>>>>>>>> DRM_SCHED_PRIORITY_MIN; i--) {
>>>>>>>>>> +        struct drm_sched_rq *rq = &sched->sched_rq[i];
>>>>>>>>>> +
>>>>>>>>>> +        if (!rq)
>>>>>>>>>> +            continue;
>>>>>>>>>> +
>>>>>>>>>> +        /* Loop this way because rq->lock is taken in 
>>>>>>>>>> drm_sched_rq_remove_entity */
>>>>>>>>>> +        spin_lock(&rq->lock);
>>>>>>>>>> +        while ((s_entity = 
>>>>>>>>>> list_first_entry_or_null(&rq->entities,
>>>>>>>>>> +                                struct drm_sched_entity,
>>>>>>>>>> +                                list))) {
>>>>>>>>>> +            spin_unlock(&rq->lock);
>>>>>>>>>> +
>>>>>>>>>> +            /* Prevent reinsertion and remove */
>>>>>>>>>> + spin_lock(&s_entity->rq_lock);
>>>>>>>>>> +            s_entity->stopped = true;
>>>>>>>>>> +            drm_sched_rq_remove_entity(rq, s_entity);
>>>>>>>>>> + spin_unlock(&s_entity->rq_lock);
>>>>>>>>>
>>>>>>>>> Well this spin_unlock/lock dance here doesn't look correct at 
>>>>>>>>> all now.
>>>>>>>>>
>>>>>>>>> Christian.
>>>>>>>>
>>>>>>>>
>>>>>>>> In what way ? It's in the same same order as in other call 
>>>>>>>> sites (see drm_sched_entity_push_job and drm_sched_entity_flush).
>>>>>>>> If i just locked rq->lock and did list_for_each_entry_safe 
>>>>>>>> while manually deleting entity->list instead of calling
>>>>>>>> drm_sched_rq_remove_entity this still would not be possible as 
>>>>>>>> the order of lock acquisition between s_entity->rq_lock
>>>>>>>> and rq->lock would be reverse compared to the call sites 
>>>>>>>> mentioned above.
>>>>>>>
>>>>>>> Ah, now I understand. You need this because 
>>>>>>> drm_sched_rq_remove_entity() will grab the rq lock again!
>>>>>>>
>>>>>>> Problem is now what prevents the entity from being destroyed 
>>>>>>> while you remove it?
>>>>>>>
>>>>>>> Christian.
>>>>>>
>>>>>> Right, well, since (unfortunately) sched_entity is part of 
>>>>>> amdgpu_ctx_entity and amdgpu_ctx_entity is refcounted
>>>>>> there is a problem here that we don't increment 
>>>>>> amdgpu_ctx.refcount when assigning  sched_entity
>>>>>> to new rq (e.g. before drm_sched_rq_add_entity) and not decrement 
>>>>>> before removing. We do it for
>>>>>> amdgpu_cs_parser.entity for example (in amdgpu_cs_parser_init and 
>>>>>> amdgpu_cs_parser_fini by
>>>>>> calling amdgpu_ctx_get and amdgpu_ctx_put). But this seems a bit 
>>>>>> tricky due to all the drm_sched_entity_select_rq
>>>>>> logic.
>>>>>>
>>>>>> Another, kind of a band aid fix, would probably be just locking 
>>>>>> amdgpu_ctx_mgr.lock around drm_sched_fini
>>>>>> when finalizing the fence driver and around idr iteration in 
>>>>>> amdgpu_ctx_mgr_fini (which should be lock protected
>>>>>> anyway as I see from other idr usages in the code) ... This 
>>>>>> should prevent this use after free.
>>>>>
>>>>> Puh, that's rather complicated as well. Ok let's look at it from 
>>>>> the other side for a moment.
>>>>>
>>>>> Why do we have to remove the entities from the rq in the first place?
>>>>>
>>>>> Wouldn't it be sufficient to just set all of them to stopped?
>>>>>
>>>>> Christian.
>>>>
>>>>
>>>> And adding it as another  condition in drm_sched_entity_is_idle ?
>>
>> Yes, I think that should work.
>
>
> In this case reverse locking order is created, In 
> drm_sched_entity_push_job and drm_sched_entity_flush lock 
> entity->rq_lock locked first and rq->lock locked second. In 
> drm_sched_fini on the other hand, I need to lock rq->lock first to 
> iterate rq->entities and then lock s_entity->rq_lock for each entity 
> to modify s_entity->stopped. I guess we could change s_entity->stopped 
> to atomic ?

Good point. But I think the memory barrier inserted by 
wake_up_all(&sched->job_scheduled); should be sufficient.

If I see this correctly we have the entity->rq_lock mainly to protect 
concurrent changes of entity->rq.

But when two CPUs both set entity->stopped to true at the same time we 
don't really care about it as long drm_sched_entity_is_idle() sees it.

Regards,
Christian.

>
> Andrey
>
>
>>
>> Christian.
>>
>>
>>>>
>>>> Andrey
>>>>
>>>>
>>>>>
>>>>>>
>>>>>> Andrey
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> Andrey
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> +
>>>>>>>>>> +            spin_lock(&rq->lock);
>>>>>>>>>> +        }
>>>>>>>>>> +        spin_unlock(&rq->lock);
>>>>>>>>>> +
>>>>>>>>>> +    }
>>>>>>>>>> +
>>>>>>>>>> +    /* Wakeup everyone stuck in drm_sched_entity_flush for 
>>>>>>>>>> this scheduler */
>>>>>>>>>> +    wake_up_all(&sched->job_scheduled);
>>>>>>>>>> +
>>>>>>>>>>       /* Confirm no work left behind accessing device 
>>>>>>>>>> structures */
>>>>>>>>>> cancel_delayed_work_sync(&sched->work_tdr);
>>>>>>>>>
>>>>>>>
>>>>>
>>

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